1. Field of the Invention
The invention relates to a process for producing a magnetoresistive transducer and more particularly a magnetoresistive transducer having perpendicular transport.
The process of the invention furthermore makes it possible to produce a large number of these transducers in the same step, which transducers may or may not be connected together.
2. Discussion of the Background
In the various fields of application of thin-film magnetoresistive materials (read head for magnetic recording, magnetometers, compasses, various types of sensors, etc.), all the components are nowadays produced with thin layers of magnetic alloys such as "Permalloy": Ni80Fe20.
The discovery of a giant magnetoresistive effect in magnetic metallic multilayers 1! was described, for example, in the document by M. N. Baibich et al., Physical Review Letters 61, p 2472, 1989 and in the document by T. Valet et al, Applied Physics Letters 61, p 3187, 1992. It is already accepted that these materials exhibit superior performance compared to those of magnetic alloys used hitherto.
The physical origin of this effect resides in the spin dependence of the scattering of carriers in magnetic metals and their alloys (see the document by A. Campbell et al., "Transport Properties of Ferromagnets" which appeared in Ferromagnetic Material, published by Wohlfarth, Amsterdam, 1982, vol 3) and in the existence of two magnetic states in these multilayers: a state in which the magnetizations of the magnetic layers all point in the same direction and which will be denoted P and the other state in which the magnetizations are alternately in one direction and then in the other and which will be denoted AP.
Switching from one state of alignment to the other causes a variation in the resistivity which may be written in the form: EQU .rho..sup.AP =.rho..sup.P (1+.alpha.) 1!
Two types of geometry are possible, the first denoted CIP (the one which has been studied most) in which the current I.sub.0 flows parallel to the layers (see FIG. 1A), and the second denoted CPP in which the current I.sub.90 flows perpendicularly to the layers (see FIG. 1B).
As has been demonstrated by theoretical work (see the document by T. Valet et al., Physical Review, B 48, No. 10, p 7099, 1993) and experimental work (see the document by W. P. Pratt, Physical Review Letters, 66, p 3060, 1991), the second configuration is potentially more promising. Nevertheless, the difficulties encountered when implementing it have limited its study.
In order to clarify this point, it is necessary to calculate not the resistivity variation .alpha. of the multilayer, which is an intrinsic variation, but that of the total resistance which is the experimentally accessible quantity.
The resistance R of an object of resistivity .rho., of cross-section S and of length l is expressed in the following manner: EQU R=.rho.l/S 2!
For a set of resistances in series, the total resistance will be the sum of all these resistances. Thus, in both the abovementioned cases (CIP and CPP), denoting by R.sub.m the resistance of the multilayer, by R.sub.f that of the current leads and by R.sub.c that resulting from the contact between the multilayer and these leads, the total resistance is given by: EQU R.sub.t =2R.sub.c +R.sub.m +2R.sub.f 3!
In the general case in which the lines of current I.sub..theta. make an angle .theta. with respect to the planes of the layers (FIGS. 1a and 1b), the resistance variation associated with the situations on switching from a P to an AP state or conversely may be written, from the formulae 1! to 3!: EQU a.sub.G =.alpha..sub.m /(1+2R.sub.f /2.sub.m +2R.sub.c /R.sub.m)4!
where .alpha..sub.m =.beta..alpha..sub.m CPP+(1-.beta.).alpha..sub.m CIP
.beta. being such that .beta.=1 if .theta.=.pi./2 and .beta.=0 if .theta.=0 ##EQU1## .phi..sub.m.sup.G.AP means resistivity in the antiparallel state of the CIP or CPP geometry for the magnetic layer, PA1 .phi..sub.m.sup.G.P means resistivity in the parallel state of the CIP or CPP geometry for the magnetic layer. PA1 the geometrical dimensions of the electrical contacts; PA1 those of the multilayer; PA1 the resistivities; and PA1 the quality of the contacts. PA1 a small drop in potential, not very compatible with applications; PA1 a measurement perturbed by the various sources of noise (especially thermal noise). PA1 L.sub.m =10 mm, L.sub.f =1 mma.sub.CIP .apprxeq..alpha..sub.m PA1 R.sub.t =R.sub.m =300 .OMEGA. PA1 e.sub.m .apprxeq.e.sub.f /10=0.1 .mu.ma.sub.CPP =.alpha..sub.m /3; PA1 we therefore get at only one third of the effect, and for the ideal case in which a.sub.CPP =.alpha..sub.m : PA1 we therefore have, in the CPP case, a resistance which is smaller by four orders of magnitude compared to the CIP case. PA1 to obtain a measurable resistance: R.sub.t PA1 to get at the maximum variation: .alpha..sub.m CPP.
It is therefore apparent from the formulae 4! that the measured variation a.sub.G is a mixture of the effects obtained in the two geometries.
In addition, these formulae show that the value of the measured effect is attenuated by the contact resistances and by the resistances of the current leads, this attenuation depending, from the formulae 2! and 3!, on:
Thus, in order to get at the intrinsic value .alpha..sub.m of the effect, which is the maximum variation which it is possible to obtain, it is necessary to decrease the resistance ratio (R.sub.f +R.sub.c)/R.sub.m and, in order to decouple the .alpha..sub.m CIP and .alpha..sub.m CPP effects, to produce a structure in which the lines of current are parallel (CIP case) or perpendicular (CPP case) to the planes of the layers.
This may be achieved in two ways, one by varying the geometrical dimensions of the various elements of the structure, the other by modifying the resistivities of its constituent parts.
In the CIP case (FIG. 1a), it is very easy to obtain the intrinsic effect .alpha..sub.m CIP. To do this, it is sufficient to increase the distance travelled by the current in the multilayer (parameter L.sub.m), into a serpentine structure for example. On the other hand, in the CPP case (FIG. 1b), this length is limited to the thickness of the multilayer and it is then necessary to vary the other parameters if it is desired to get close to the intrinsic variation .alpha..sub.m CPP.
Another major problem resulting from this geometry is the low value of the resistance due to the multilayer.
This has two consequences:
In order to specify an order of magnitude of these values, assuming that the contact resistance is perfect and that the lines of current are virtually parallel to the plane of the layers in the CIP case and virtually perpendicular in the CPP case, let us compare them in the two geometries and take the case in which the electrical contacts are made of copper and in which the multilayer consists of permalloy and copper (see the document by T. Valet et al., Applied Physics Letters, 61, p 3187, 1992).
We then have, from FIGS. 1a and 1b and the formulae 2! to 4!: EQU a.sub.CIP =.alpha..sub.m CIP/(1+2.rho..sub.f L.sub.f S.sub.m /.rho..sup.p.sub.m L.sub.m S.sub.f) EQU a.sub.CPP =.alpha..sub.m CPP/(1+2.rho..sub.f e.sub.f S.sub.m /.rho..sup.p.sub.m e.sub.m S.sub.f)
where .rho..sub.f .apprxeq.3.mu..OMEGA. cm, .rho..sub.m .apprxeq.30.mu..OMEGA. cm
and in the case where S.sub.f .apprxeq.S.sub.m .apprxeq.1 .mu.m.sup.2
we obtain EQU a.sub.CIP =.alpha..sub.m CIP/(1+0.2L.sub.f /L.sub.m) EQU a.sub.CPP =.alpha..sub.m CPP/(1+0.2e.sub.f /e.sub.m)
In the CIP case, we have, easily:
In the CPP case, typical values are:
R.sub.t =R.sub.m =0.03 .OMEGA.
In the CPP case, there are therefore two problems to be solved which are:
One of the solutions developed for remedying these various problems uses superconducting contacts made of niobium (see the document by W. P. Pratt et al., Physical Review Letters, 66, p 3060, 1991). Although this approach makes it possible to get round the resistances of the current leads, it requires the use of low temperature (4.2 K.), which limits the advantage thereof for the abovementioned applications.
A more technological approach has been developed and described in the document by M. A. M. Gijs et al., Applied Physics Letters, 63, p 111, 1993. This approach consists in reducing the cross-section S.sub.m of the multilayer using the techniques available in microlithography.
FIG. 2a illustrates the structure fabricated by this technique. Firstly, by masking and exposure to ultraviolet radiation, a square pattern is printed in a photosensitive resin deposited on the stack of materials in which it is desired to machine the block 1 (here, a multilayer 2 deposited on a gold layer 3). After machining the block, the resin is dissolved in an acetone bath. Next, the whole assembly is covered with a photosensitive material 4 (polyimide) which serves as an insulation. Next, an aperture, obtained by exposure to ultraviolet radiation located above the pillar, is made in this insulation. Finally, immediately after having cleaned the thus-revealed surface, a gold layer 5 is deposited on the entire structure (contacting).
In this technique, there is a strong correlation between the cross-section of the pillars, the thickness of the conductive layers surrounding the latter and that part of the measured resistance coming from this column. The results obtained show that, for a conductor thickness of less than two micrometres and for cross-sections of less than about ten square micrometres, that part of the resistance of the column may become very low. In order to remedy this, a trench 6 is made on either side of the block, as depicted in FIG. 2b (view from above of FIG. 2a). The measured resistance is then that of the pillar, and is independent of the values of the thicknesses and cross-sections.
However, this technology has many drawbacks.
On the one hand, the structure thus fabricated has a high contact resistance due to the necessity of using a photosensitive material as insulation which, when cleaning the top of the blocks, is pulled off and redeposited on this surface.
On the other hand, the necessity, when opening a window in the top of the contacting studs or when producing the trench, of being positioned with an accuracy at least equal to that of their diameters, makes this step difficult for cross-sections of a few square micrometres. In addition, it appears to be difficult to cut a trench vertically in line with the pillar deviations .DELTA.x and .DELTA.y in FIG. 2b).